82 o tf T L I N E S OF geology. 



Gases and vapours escaping from the igneous mass, while 

 in the act of consolidation, often leave it full of vesicular ca- 

 vities ; into these, mineral matter is occasionally carried by 

 the filtration of water, and deposited in crystaline forms, or 

 in concentric layers. 



The eruptions of volcanoes are often preceded by earth- 

 quakes ; these tend and tear the rocks of former ejections, 

 which may also have been rent by their own contraction on 

 Cooling. The fissures formed in either way are filled up by 

 the matter of subsequent eruptions, in the form of dykes and 

 veins, and these may penetrate into rocks of other origin, 

 and cause them to be traversed by similar joints, or inter- 

 mixed with intercalated rocks* 



Besides a regular flow of liquid matter, over the level edge 

 ©f a crater, it frequently happens that the edge is not level 

 or has been split into fissures. In such cases, instead of 

 spreading uniformly, and assuming a structure resembling 

 stratification, the lava makes its way in streams ; these, on 

 cooling, form overlying masses, and should they enter into 

 the sea, assume the columnar structure which we have ob- 

 served in the trapp and basaltic rocks. 



The earthquakes which precede volcanic eruptions, often 

 raise the ground in which they prevail, and thus when a vent 

 is found, the crater makes its appearance in the middle of a 

 mountain formed of rocks of an origin wholly distinct. Such 

 mounts may not only be formed of land already in the dry 

 state, but may be raised from the depths of the sea. In this 

 way the curious but frequent spectacle is presented, of form- 

 ations of aqueous origin, overlaid, traversed, and penetrated 

 by volcanic rocks. 



The ejections of volcanoes are not limited to matter in a 

 state of igneous fusion. Solid bodies are often thrown out 

 with great violence in the form of fine powder, sand, angular 

 fragments of the size of gravel, or pebbles, and even large 



